Semiconductor memory device and manufacturing method thereof

ABSTRACT

After an ONO film in which a silicon nitride film ( 22 ) formed by a plasma nitriding method using a plasma processor having a radial line slot antenna is sandwiched by silicon oxide films ( 21 ), ( 23 ), a bit line diffusion layer ( 17 ) is formed in a memory cell array region ( 11 ) by an ion implantation as a resist pattern ( 16 ) taken as a mask, then lattice defects are given to the silicon nitride film ( 22 ) by a further ion implantation. Accordingly, a highly reliable semiconductor memory device can be realized, in which a high quality nitride film is formed in a low temperature condition, in addition, the nitride film can be used as a charge trap film having a charge capture function sufficiently adaptable for a miniaturization and a high integration which are recent demands.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of Ser. No. 11/878,296, filed Jul. 23,2007, which is a division of Ser. No. 11/065,305, filed Feb. 25, 2005,which is a continuation of international application No. PCT/JP03/11109filed Aug. 29, 2003, which is based on Japanese Application No.2002-255528 filed Aug. 30, 2002.

TECHNICAL FIELD

The present invention relates to a semiconductor memory device and amanufacturing method thereof, which has a gate insulating film includinga nitride film and capable of holding information by storing the chargein the nitride film.

BACKGROUND ART

In recent years, a multitude of so-called flash memories are used as arewritable semiconductor memory device. The flash memory using afloating gate prevails among them, however, it is difficult for thememory of this type to allow a tunnel insulating film to be thinner,which is an obstacle for the mass storage. Hence, a multi-value memorycell is thought out, in which a threshold value of a transistor isvaried by controlling a charge amount to the floating gate when writing,and multi-value data is allowed to be stored in one memory cell.

A new multi-value memory cell having an MNOS structure or a SONOSstructure, different from the floating gate type memory cell, isproposed, which applies, to the gate insulating film just under a gateelectrode, a two-layered structure of an oxide film/an nitride film(ON), namely, the structure that the nitride film is stacked on theoxide film as seen from a substrate, or a three-layered structure of theoxide film/the nitride film/the oxide film (ONO), namely, the structurethat the nitride film and the oxide film are sequentially stacked on theoxide film seen from the substrate, and which stores the charge locallyin the nitride film in the vicinity of respective source/drain of thetransistor, resultingly storing 2-bits data with respect to one memorycell.

Such multi-value memory cell has a simpler structure than that of thefloating gate-type memory cell, and has an advantage that a cell areaper bit is about ½, compared with the floating-gate type memory cell.And further, a memory cell having a memory cell array structure andbeing advantageous to a miniaturization are studied, which does not havea contact hole for a bit line in each transistor even though it is a NORtype memory, by using a source/drain as a bit line (an embedded bitline), namely, by forming the bit line under a word line.

In the memory cell having the aforementioned MNOS structure or SONOSstructure, in order to form the gate insulating film, first, after athin lower oxide film (tunnel oxide film) having a film thickness ofapproximately 7 nm is formed by a thermal oxidation method, a nitridefilm having a film thickness of approximately 10 nm is deposited by aCVD method to form an ON film. In the case of the SONOS structure, anupper oxide film is further formed by thermally oxidizing an upperportion of the nitride film to form an ONO film in which the nitridefilm is sandwiched between upper and lower oxide films.

When the above-mentioned nitride film is formed by the CVD method, aheat of approximately 650° C. to 850° C. is applied. And further, inorder to form the upper oxide film by the thermal oxidation of thenitride film, a heat treatment of 1000° C. or more is required. Inaddition, in order to form the high-quality lower oxide film, atemperature condition of 900° C. or more is required.

Accordingly, in order to form the ON film or the ONO film by the CVDmethod, there exists a problem that the high temperature heat treatmentfor a long time is fundamental and that a matching property is extremelylow with respect to the miniaturization of a semiconductor element,especially a transistor, of a peripheral circuit region of a memorycell. In addition, in the memory cell structure to which an embedded bitline is used, an impurity of the bit line is diffused by the hightemperature heat treatment. If the bit line is formed after the ON filmor the ONO film is formed to avoid the diffusion, there is a problemthat damage is generated in the ON film or the ONO film and the decreaseof a withstand voltage is caused.

Since the nitride film (CVD nitride film) formed by the CVD method hasmany N holes constituting charge trap centers, the film is used for acharge trap film of the transistor of the MNOS structure or the SONOSstructure and the like. However, the formation of the CVD nitride filmrequires the high temperature as described above, and further, in thecase of the highly integrated MNOS structure or the SONOS structure andthe like which stores 2 bits per cell by performing a charge injectiononly to respective edge portions of the source/drain switched between aread-out occasion and a rewrite occasion, the charge trap centers existat portions where the charge injection is desired to be avoided in thenitride film in which the N holes are formed almost uniformly such asthe CVD nitride film, so that inconvenience for a device operation iscaused.

For example, if electrons are injected and stored to portions other thanportions to which electrons or holes are injected and stored in thenitride film (in the vicinity of the edge of the drain), especially thecenter portion of a channel, there exists a problem that a thresholdvalue of the transistor increases and a margin in the low state ofthreshold value decreases regardless of the injection state of theelectrons or holes to the vicinity of the edge of the drain.

The present invention is made in view of the above problems and anobject thereof is to provide a highly reliable semiconductor memorydevice and a manufacturing method thereof, in which a high qualitynitride film is formed in a low temperature condition, in addition, thenitride film can be used as a charge trap film having a charge capturefunction sufficiently adaptable for a miniaturization and a highintegration which are recent demands.

SUMMARY OF THE INVENTION

As a result of repeating the earnest study, the present inventor thoughtout the following aspects of the present invention.

A semiconductor memory device of the present invention includes asemiconductor substrate, an insulating film formed on the semiconductorsubstrate and having a nitride film including a charge capture function,a gate electrode formed on the semiconductor substrate through theinsulating film, and a pair of impurity diffusion layers formed on thesemiconductor substrate, and the nitride film includes charge trapcenters in which lattice defects are formed at a specified portionthereof.

A manufacturing method of a semiconductor memory device of the presentinvention includes the steps of forming an insulating film including anitride film formed on a semiconductor substrate by a plasma nitridingmethod, forming a pair of impurity diffusion layers with an impurityintroduced into a surface layer of the semiconductor substrate, formingcharge trap centers with lattice defects given to portions correspondingto at least on the impurity diffusion layers in the nitride film beforeor after the formation of the impurity diffusion layer, and forming agate electrode so as to cross the impurity diffusion layer through theinsulating film.

Another aspect of the manufacturing method of the semiconductor memorydevice of the present invention includes the steps of forming aninsulating film including a nitride film formed on a semiconductorsubstrate by a plasma nitriding method, forming a gate electrode on theinsulating film, forming a pair of impurity diffusion layers with animpurity introduced into a surface layer of the semiconductor substratewith, at least, the gate electrode taken as a mask, and forming chargetrap centers with lattice defects given to a portion corresponding to,at least, on the impurity diffusion layer in the nitride film with, atleast, the gate electrode taken as a mask before or after the formationof the impurity diffusion layer.

In the aforementioned manufacturing method, the nitride film is formedby performing a nitriding processing with the plasma excited by amicrowave in an atmosphere of a source gas including nitrogen atoms forgenerating a nitride radical.

In the aforementioned manufacturing method, the lattice defects areformed in the nitride film by introducing an impurity into the nitridefilm or performing a radio frequency processing using an inert gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A to FIG. 1D are schematic sectional views showing a manufacturingmethod of a semiconductor memory device including an embedded bit linetype SONOS transistor according to a first embodiment in order ofprocess;

FIG. 2A to FIG. 2D are schematic sectional views showing themanufacturing method of the semiconductor memory device including theembedded bit line type SONOS transistor according to the firstembodiment followed by FIG. 1D in order of process;

FIG. 3A and FIG. 3B are schematic sectional views showing themanufacturing method of the semiconductor memory device including theembedded bit line type SONOS transistor according to the firstembodiment followed by FIG. 2D in order of process;

FIG. 4 is a schematic view showing a schematic configuration of a plasmaprocessor including a radial line slot antenna used for someembodiments;

FIG. 5 is a schematic sectional view showing a main process in amanufacturing method of a semiconductor memory device of a modificationexample 1 in the first embodiment;

FIG. 6A to FIG. 6C are schematic sectional views showing main processesin a manufacturing method of a semiconductor memory device of amodification example 2 in the first embodiment;

FIG. 7A to FIG. 7D are schematic sectional views showing a manufacturingmethod of a semiconductor memory device including a MONOS transistoraccording to a second embodiment in order of process;

FIG. 8A and FIG. 8B are schematic sectional views showing themanufacturing method of the semiconductor memory device including theNOMOS transistor according to the second embodiment followed by FIG. 7Din order of process;

FIG. 9A to FIG. 9D are schematic sectional views showing main processesof an embedded bit line type SNOS transistor according to a thirdembodiment;

FIG. 10 is a schematic sectional view showing a main process of asemiconductor memory device including an embedded bit line type SNOStransistor according to another example of the third embodiment;

FIG. 11A and FIG. 11B are schematic sectional views showing mainprocesses of a semiconductor memory device including an embedded bitline type SNOS transistor according to still another example of thethird embodiment;

FIG. 12A and FIG. 12B are schematic sectional views showing mainprocesses of a semiconductor memory device including an MNOS transistoraccording to still another example of the third embodiment;

FIG. 13A to FIG. 13D are schematic sectional views showing mainprocesses of a semiconductor memory device including an embedded bitline type SNOS transistor according to a fourth embodiment;

FIG. 14 is a schematic sectional view showing a main process of asemiconductor memory device including an embedded bit line type SNStransistor according to another example of the fourth embodiment;

FIG. 15A and FIG. 15B are schematic sectional view showing mainprocesses of a semiconductor memory device including an embedded bitline type SNS transistor according to still another example of thefourth embodiment;

FIG. 16A and FIG. 16B are schematic sectional view showing mainprocesses of a semiconductor memory device including an MNS transistoraccording to still another example of the fourth embodiment;

FIG. 17 is a schematic sectional view showing an RF processing in amanufacturing method of a semiconductor memory device according to afifth embodiment; and

FIG. 18A and FIG. 18B are schematic sectional views showing anotherforming method of a plasma silicon nitride film according to someembodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Basic Gist of thePresent Invention

First, the basic gist of the present invention will be described.

In the present invention, in addition to manufacturing a semiconductormemory device in a low temperature condition through the entiremanufacturing process, specifically, in the low temperature condition at600° C. or less with respect to the processing having the heat for morethan several minutes except the processing of rapid temperature increaseand decrease within approximately one minute such as RTA and the like,in consideration of obtaining a dense and good quality nitride film (acharge trap film), a nitriding processing (a plasma nitriding method)using a nitride radical formed by excited plasma is employed instead ofa CVD method requiring a high temperature, when the nitride film isformed. And further, not only the nitride film but also an oxide filmunder the nitride film in an ON film, and oxide films of upper and lowerlayers with respect to the nitride film in an ONO film are similarlyformed by a plasma processing (a plasma oxidation method), so that thefurther reduction of thermal budget can be possible and a matchingproperty with a peripheral circuit region is improved.

The plasma nitriding method is a method such that the nitridingprocessing is performed by generating the nitride radical (NH*radical orN*radical) with the plasma excided by a microwave in an atmosphere of asource gas including nitrogen atoms, for example, one kind selected froma NH₃ gas, a mixed gas of N₂ and H₂, and N₂ gas, or a mixed gas of theNH₃ gas and N₂ gas, or a mixed gas of the NH₃ gas, the N₂, and the H₂.According to this method, the dense and good quality plasma nitride filmcan be obtained at a low temperature of approximately 200° C. to 600° C.Though the plasma nitride film is formed by the plasma nitriding methodalone or by a series of processes including the plasma nitriding method,in the following description, it will be written that it is formed “bythe plasma nitriding method” for convenience.

Since the plasma nitriding film is the dense and good quality nitridingfilm, it has extremely little lattice defect in the whole surface,therefore it has little N hole, so that it is an excellent nitride filmwhen used on a portion to which the charge storage is required to beavoided. At the same time, however, the film inevitably has a difficultyin storing the charge on a specified portion to which the charge storageis required (for example, the vicinity of the edge of a drain).

Considering that the charge storage is performed on the specifiedportion when the dense and good quality nitride film using the plasmanitriding method, the present inventor found out that charge trapcenters are formed only on a specified portion by selectively givinglattice defects to the specified portion of the plasma nitride film, forexample, to the vicinity of the edge of the drain, besides forming thenitride film by the plasma nitriding method. In this case, it goeswithout saying that the lattice defects can be given to the wholesurface of the plasma nitride film.

As a method of giving the lattice defects to the specified portion ofthe plasma nitride film, to introduce an impurity to the specifiedportion by an ion implantation and to perform a radio frequencyprocessing (RF processing) using an inert gas to the specified portionare effective. Specifically, a resist pattern having openings only atspecified portions of the plasma nitride film is formed, and the ionimplantation or the RF processing is performed using the resist patternas a mask.

In this case, when a nitride film charge-storage memory including anembedded bit line type SONOS/SNOS/SNS transistors, i.e., including asource/drain used also for the embedded bit line and having a channelparallel to a word line (gate electrode) is fabricated, a resist patternfor forming the source/drain is also used as a mask. On the other hand,when a nitride film charge-storage memory including conventionalMONOS/MNOS/MNS transistors, i.e., including the source/drain formed by aself-alignment with the word line (gate electrode), and having a channelvertical to the word line is fabricated, to use the gate electrode forforming the source/drain also as a mask is suitable because it leads toa reduction of processes.

Specific Embodiments

Based on the aforementioned basic gist of the present invention,specific embodiments will be described below.

First Embodiment

In the present invention, an embedded bit line type SONOS transistor asa semiconductor memory device will be disclosed. For convenience, astructure of the transistor will be explained with a manufacturingmethod thereof.

In the semiconductor memory device, a SONOS transistor in a memory cellarray region is a planar type and a CMOS transistor is formed in aperipheral circuit.

FIG. 1A to FIG. 1D and FIG. 2A to FIG. 2D are schematic sectional viewsshowing a manufacturing method of the semiconductor memory deviceincluding an embedded bit line type SONOS transistor according to thepresent invention in order of process, and FIG. 3B is a schematic planview corresponding to FIG. 3A. In FIG. 3A and FIG. 3B, the memory cellarray region (core region) alone is shown, a cross section taken alongthe line I-I of FIG. 3B corresponds to a core X of FIG. 3A, and a crosssection taken along the line II-II corresponds to a core Y of FIG. 3Arespectively.

First, as shown in FIG. 1A, wells 3, 4 are formed in a peripheralcircuit region 12. Specifically, a silicon oxide film 2 having a filmthickness of approximately 20 nm is formed on a surface of a p-typesilicon semiconductor substrate 1. By patterning the silicon oxide film2 by means of a photolithography and a subsequent dry etching, a mask isformed, of which shape has openings at predetermined portions of theperipheral circuit region 12. After ion implanting an n-type impurity,phosphor (P) in this case, into the openings using this mask, theimpurity is thermally diffused by an annealing treatment to form eachwell 3 in the peripheral circuit region 12. It is also preferable toion-implant a p-type impurity, for example boron (B), further in thewell 3 to form a triple well 4.

Subsequently, as shown in FIG. 1B, element isolation structures areformed in the peripheral circuit region 12. In this process, field oxidefilms 14 are formed in the peripheral circuit region 12 by a so-calledLOCOS method to demarcate element active regions. It is also preferableto perform, for example, an STI (Shallow Trench Isolation) method thatforms trenches in element isolation regions of the semiconductorsubstrate and fills the trenches with insulators to form the elementisolation structures, instead of the LOCOS method.

Specifically, a SiN film (not shown) is formed to have a film thicknessof approximately 100 nm on the silicon oxide film 2 and processed bypatterning to be a shape which has openings only at the elementisolation regions of the peripheral region 12. Portions exposed from theopenings of the silicon oxide film 2 are oxidized and the thick fieldoxide films 14 are formed only at the element isolation regions todemarcate the element active regions.

Subsequently, as shown in FIG. 1C, after removing the silicon oxide film2, a lower silicon oxide film 21 to be a tunnel insulating film, asilicon nitride film 22 to be a charge trap film, and an upper siliconoxide film 23 respectively having film thicknesses of approximately 7nm, 10 nm, and 10 nm are sequentially formed as an ONO film 15.

The silicon nitride film 22 is formed by a plasma nitriding method witha microwave excited.

Specifically, a plasma nitriding is performed by means of a plasmaprocessor including a radial line slot antenna as shown in FIG. 4. Theplasma processor 100 is composed of a gate valve 102 continuouslyconnected to a cluster tool 101, a processing chamber 105 which canhouse a susceptor 104 on which a processing object W (semiconductorsubstrate 1 in the present embodiment) is mounted and has a coolingjacket 103 cooling the processing object W when the plasma processing, ahigh-vacuum pump 106 connected to the processing chamber 105, amicrowave supply source 110, an antenna member 120, a radio-frequencypower supply for bias 107 and a matching box 108 composing an ionplating with the antenna member 120, gas supply systems 130, 140including gas supply rings 131, 141 and a temperature control unit 150performing the temperature control of the processing object W.

The microwave supply source 110 is made of, for example, magnetron, andcan usually generate 2.45 GHz microwave (for example, 5 kW). After that,the transmission mode of the microwave is converted to a TM mode, a TEmode, TEM mode or the like by a mode converter 112.

The antenna member 120 includes a temperature-adjusting plate 122, ahousing member 123 and a dielectric plate. The temperature-adjustingplate 122 is connected to a temperature controller 121, and the housingmember 123 houses a wavelength shortening material 124 and a slotelectrode (not shown) touching the wavelength shortening material 124.The slot electrode is called as a radial line slot antenna (RLSA) or anultrahigh efficiency flat antenna. In the present embodiment, othertypes of antennas, for example, a single layer waveguide flat antenna, adielectric substrate parallel plane slot antenna and the like areapplicable.

By means of the above structured plasma processor, the nitridingprocessing is performed at a temperature condition of 450° C. using Ar,N₂, and H₂ as a source gas, and approximately 20 nm of the surface layerof the oxide film is substituted by the nitride film of which filmthickness is 27 nm, and the silicon nitride film 22 is formed on thelower silicon oxide film 21.

In the same way, the lower silicon oxide film 21 and the upper siliconoxide film 23 are formed by the plasma oxidizing. In this case, theoxidizing is performed by using Ar and O₂ as the source gas andgenerating an oxygen radical (O*), and approximately 10 nm of thesurface layer of the oxide film is substituted by the oxide film ofwhich film thickness is approximately 20 nm at the temperature conditionof 450° C. Note that the lower silicon oxide film 21 and the uppersilicon oxide film 23 can be formed by a thermal oxidation method or aCVD oxidation method instead of the plasma oxidizing.

Next, as shown in FIG. 1D, a photoresist is coated on the whole surfaceand a resist pattern 16 is formed by the photolithography, of whichshape has openings only at portions where bit lines are formed in amemory cell array region 11.

Then, after bit line diffusion layers 17 are formed in the memory cellarray region 11 by ion implanting with the resist pattern 16 taken as amask, lattice defects are given to the silicon nitride film 22 by afurther ion implantation.

Specifically, first, as shown in FIG. 2A, an n-type impurity, arsenic(As) in this case, is ion implanted with the resist pattern 16 taken asthe mask in a condition that an acceleration energy is 70 keV, and adose amount is 2.0×10¹⁵/cm² to form the bit line diffusion layers 17 tobe sources/drains on the surface layer of the semiconductor substrate 1,corresponding to positions of the ONO film 15 exposed from the openings16 a of the resist pattern 16.

Next, as shown in FIG. 2B, the arsenic is ion implanted in a conditionthat the acceleration energy is 10 keV to 20 keV, and the dose amount isfrom 1.0×10¹²/cm² to 2.0×10¹⁵/cm², from a direction inclined at apredetermined angle, a tilt angle of 45° in this case, with respect todirection vertical to the surface of the semiconductor substrate 1, withthe resist pattern 16 taken as the mask again, and lattice defects 22 aare selectively given to the portions corresponding to the openings 16 aof the resist pattern 16 of the silicon nitride film 22, namely, theportions corresponding to upper portions of the bit line diffusionlayers 17 to form the charge trap centers. Owing to the ion implantationwith the tilt angle given, a memory cell structure such that hotelectrons are chiefly injected into the place approximately 30 nm˜50 nminside the channel from the joint portion of the bit line diffusionlayer 17 can be realized.

It is related, for example, in “Characterization of Channel Hot ElectronInjection by the Subthreshould Slope of NROM™ Device” in IEEE ELECTRONDEVICE LETTERS, VOL. 22, No. 11 issued in November 2001, that an actualdevice characteristic corresponds to a simulation result when the hotelectrons are injected into the place 40 nm inside the channel from thejoint portion of the bit line diffusion layer.

In the ion implantation of the impurity which gives damage to thesilicon nitride film 22, it is preferable to select an accelerationenergy to damage as little as possible the lower silicon oxide film 21as being a base tunnel insulating film. In the present embodiment, adensity peak of arsenic at the time of the ion implantation is set to beexist in the vicinity of an interface between the silicon nitride film22 and the upper silicon oxide film 23, and the rate of the arsenicpenetrating to the lower silicon oxide film 21 is set to be not morethan 15% of the entire ion implantation amount.

In the present embodiment, it is explained as an example that after theion implantation for forming the bit line diffusion layers 17 isperformed, the ion implantation for giving the lattice defects 22 a tothe silicon nitride film 22 is performed, however, the former ionimplantation can be performed after the latter ion implantation,reversing the order.

Subsequently, as shown in FIG. 2C, after the resist pattern 16 isremoved by an ashing treatment or the like, the ONO film 15 on theperipheral circuit region 12 is removed by patterning. Then, a gateinsulating film is formed on the peripheral circuit region 12. In thiscase, two kinds of gate insulating films having different filmthicknesses 31, 32 are formed.

Specifically, a first thermal oxidation at 1000° C. is performed to formsilicon oxide films of which film thickness is approximately 8 nm inrespective element active regions of the peripheral circuit region 12.After the photolithography and a HF processing are performed, a secondthermal oxidation at 1000° C. is performed to form silicon oxide film ofwhich film thickness is approximately 10 nm. Resultingly, the gateinsulating film 31 having a film thickness of approximately 10 nm isformed on one element active region (region of the well 3), and the gateinsulating film having a film thickness of approximately 13 nm is formedon the other element active region (region of the well 4) respectively.

Subsequently, gate electrodes 18 are pattern-formed in the memory cellarray region 11 and the peripheral circuit region 12 respectively.

Specifically, as shown in FIG. 2D, a stacked film 19 composed of apolycrystalline silicon film respectively having a film thickness ofapproximately 100 nm and a tungsten silicide film having a filmthickness of approximately 150 nm are stacked-formed. Then, as shown inFIG. 3A and FIG. 3B, by patterning the stacked film 19 and the ONO film15 (and the gate insulating films 31, 32) using the photolithography anda sequential dry etching, the gate electrode 18 crossing (in this case,orthogonal to) the bit line diffusion layer 17 through the ONO film 15and functioning as a word line is formed in the memory cell array region11, and a gate electrode (not shown) extended through the gateinsulating films 31, 32 on the semiconductor substrate 1 is formed inthe peripheral circuit region 12 respectively.

Subsequently, the sources/drains are formed, specifically, a p-typesource/drain in which a p-type impurity is ion implanted into the well3, and an n-type source/drain in which an n-type impurity is ionimplanted into the well 4 (both not shown) are formed in the peripheralcircuit region respectively.

After that, a SONOS transistor is completed through the formation of aninterlayer film, a wiring layer, a contact hole and the like (all notshown).

The SONOS transistor operates as follows.

In the vicinity of the bit line diffusion layer 17 functioning as thedrain, channel hot electrons are generated when writing data and hotholes are generated when erasing data by a band-to-band tunneling. Theelectrons and holes generated at this time are captured in the siliconnitride film 22. In addition, when the data is read out, functions ofsource and drain are switched with each other, and the difference ofthreshold value caused by the difference of positive and negative amountof the captured charge is detected.

As described above, according to the present embodiment, a highlyreliable SONOS transistor can be realized, in which the high qualitysilicon nitride film 22 is formed by the plasma nitriding method in alow temperature condition, in addition, the silicon nitride film 22 canbe used as a charge trap film having a charge capture functionsufficiently adaptable for a miniaturization and a high integrationwhich are recent demands.

Modification Example

Here, some modification examples of the present embodiment areexplained. In these modification examples, a semiconductor memory deviceincluding a embedded bit line type SONOS transistor are illustrated inthe same way as the present embodiment, however, these examples differfrom the present embodiment in a part of the main process.

Modification Example 1

FIG. 5 is a schematic sectional view showing a main process in amanufacturing method of a semiconductor memory device of a modificationexample 1.

In the modification example 1, after bit line diffusion layers 17 areformed through the respective processes of FIG. 1A to FIG. 1D in thesame way as the present embodiment, openings 16 a of a resist pattern 16are enlarged (receding) with a resist trimming performed to the resistpattern 16 to form openings 16 b which are broader than the width of thebit line diffusion layers 17, as shown in FIG. 5.

Then, an ion implantation of an impurity (arsenic in this case) isperformed from a direction vertical to the surface of the semiconductorsubstrate 1 with the resist pattern 16 having the enlarged openings 16 btaken as a mask in the same way as forming the bit line diffusion layers17, and lattice defects 22 a are given to a silicon nitride film 22 toform charge trap centers.

In this case, since the width of the opening 16 b is broader than thebit line diffusion layer 17, an excellent memory cell structure can berealized, such that hot electrons are injected into the placeapproximately 30 nm to 50 nm inside the channel from the joint portionof the bit line diffusion layer 17 even by the ion implantation with anordinary angle (tilt angle θ (zero)°).

After that, the semiconductor memory device is completed through therespective processes of FIG. 2C to FIG. 3A, FIG. 3B in the same way asthe present embodiment.

According to the present modification example, a reliable semiconductormemory device can be realized, in which the high quality silicon nitridefilm 22 is formed by the plasma nitriding method in the low temperaturecondition, in addition, the silicon nitride film 22 can be utilized as acharge trap film having a charge capture function sufficiently adaptablefor a miniaturization and a high integration which are recent demands.

Modification Example 2

FIG. 6A to FIG. 6C are schematic sectional views showing main processesof a manufacturing method of a semiconductor memory device of themodification example 2.

In the modification example 2, after element active regions aredemarcated in a peripheral circuit region 12 through the respectiveprocesses of FIG. 1A and FIG. 1B in the same way as the presentembodiment, a photoresist is coated on the whole surface before formingan ONO film 15 as shown in FIG. 6A, a resist pattern 16 is formed by aphotolithography, of which shape has openings only at portions where bitlines are formed in a memory cell array region 11, and bit linediffusion layers 17 are formed by an ion implantation with the resistpattern taken as a mask.

Subsequently, as shown in FIG. 6B, after the resist pattern 16 isremoved by the ashing treatment or the like and a silicon oxide film 2is removed, an upper silicon oxide film 21, a silicon nitride film 22 bythe aforementioned plasma nitriding method, then a lower silicon oxidefilm 23 are sequentially formed to be the ONO film 15.

Subsequently, as shown in FIG. 6C, the photoresist is coated on thewhole surface, and a resist pattern 51 is formed by thephotolithography, of which shape has openings only at portions where bitlines are formed in the memory cell array region 11. In this case asshown in the drawing, it is preferable that the resist pattern 51 isformed to be a shape having openings 51 a of which width is broader thanthe bit line diffusion layer 17.

Then, the ion implantation is performed with the resist pattern 51 takenas a mask, and the lattice defects 22 a are given to form charge trapcenters.

Specifically, arsenic is ion implanted from a direction vertical to thesurface of the semiconductor substrate 1 in a condition that anaccelerator energy is from 10 keV to 20 kev and a dose amount is from1.0×10¹²/cm² to 2.0×10¹⁵/cm², and lattice defects 22 a are selectivelygiven to portions corresponding to the openings 51 a of the resistpattern 51 of the silicon nitride film 22, namely, corresponding toupper portions of the bit line diffusion layers 17 of the siliconnitride film 22.

It is also preferable that a resist pattern having openings of whichwidths are almost same as the bit line diffusion layer 17 is formed,instead of forming the resist pattern 51 having broader openings 51 athan the width of the bit line diffusion layer 17 as the presentmodification example, and that the ion implantation is performed from adirection inclined at a predetermined angle (for example, a tilt angleof 45°) with respect to the direction vertical to the surface of thesemiconductor substrate 1 in the same way as the present embodiment.

After that, a semiconductor memory device is completed through therespective processes of FIG. 2C to FIG. 3A, and FIG. 3B in the same wayas the present embodiment.

According to the present modification example, a reliable semiconductormemory device can be realized, in which a high quality silicon nitridefilm 22 is formed by the plasma nitriding method in a low temperaturecondition, in addition, the silicon nitride film 22 can be utilized as acharge trap film having a charge capture function sufficiently adaptablefor a miniaturization and a high integration which are recent demands.

Furthermore, according to the present modification example, the ionimplantation for forming the embedded bit line 17 is performed beforethe ONO film 15 is formed so that damage generated by the impuritypenetrating through the ONO film 15 can be prevented and sufficientwithstand voltage between a bit line and a word line can be secured.

Second Embodiment

In the present embodiment, as a semiconductor memory device, atransistor having a conventional MONOS structure which is not anembedded bit line type transistor, in which element isolation structuresare formed in a memory cell array region will be disclosed. Note thatthe structure of the transistor is explained with a manufacturing methodthereof for convenience.

The semiconductor memory device is one in which a MONOS transistor of amemory cell region has a channel vertical to a word line, and a CMOStransistor is formed in a peripheral circuit region.

FIG. 7A to FIG. 7D, FIG. 8A and FIG. 8B are schematic sectional viewsshowing a manufacturing method of the semiconductor memory deviceincluding the MONOS transistor according to the present embodiment inorder of process.

In the same way as FIG. 3A, a portion denoted by a core X in the leftside of the memory cell array region (core region) 11 shows a crosssection along a longitudinal direction of a gate electrode, and aportion denoted by a core Y in the right side shows a cross sectionperpendicular to the longitudinal direction of the gate electrode inrespective drawings. With respect to structural members and the likecorresponding to the first embodiment, the same numerals are given andthe explanations thereof are omitted.

First, as shown in FIG. 7A, wells 3, 4 are formed in a peripheralcircuit region 12 in the same way as the first embodiment.

Subsequently, element active regions are demarcated respectively in thememory cell array region 11 and in the peripheral circuit region 12 by aso-called LOCOS method.

Specifically, a SiN film (not shown) is formed to have a film thicknessof 10 nm on a silicon oxide film 2. The SiN film is patterned andprocessed to a shape having openings only at respective elementisolation regions of the memory cell array region 11 and the peripheralcircuit region 12. Then, portions exposed from the openings of thesilicon oxide film 2 are oxidized and thick field oxide films 14 areformed only at the element isolation regions to demarcate element activeregions.

Subsequently, as shown in FIG. 7B, after the silicon oxide film 2 isremoved, a lower silicon oxide film 21 to be a tunnel insulating film, asilicon nitride film 22 to be a charge trap film, and an upper siliconoxide film 23 respectively having film thicknesses of approximately 7μm, 10 μm and 10 μm are sequentially formed as an ONO film 15. Thesilicon nitride film 22 is formed by the aforementioned plasma nitridingmethod in the same way as the first embodiment, and the lower siliconoxide film 21 and the upper silicon oxide film 23 are formed by a plasmaoxidizing or a thermal oxidation method or a CVD oxidation method.

Subsequently, after the ONO film 15 of the peripheral circuit region 12is removed by patterning the ONO film 15, two kinds of gate insulatingfilms 31, 32 having different film thicknesses are formed in theperipheral circuit region 12 by two times thermal oxidation.

Subsequently, as shown in FIG. 7C, after a stacked film composed of apolycrystalline silicon film and a tungsten silicide film is formed,gate electrodes 53 are pattern-formed on the ONO film in the memory cellarray region 11 and on the gate insulating films 31, 32 in theperipheral circuit region 12 by patterning the stacked film (and thegate insulating films 31, 32) in the same way as the first embodiment.

Subsequently, as shown in FIG. 7D, a photoresist is coated on the wholesurface, and a resist pattern 52 is formed by a photolithography, ofwhich shape covers the peripheral circuit region 12 and has openings inthe memory cell array region 11.

Then, after sources/drains 54 are formed in the memory cell region 11 byan ion implantation with the resist pattern 52 and the gate electrodes53 taken as masks, lattice defects are given to the silicon nitride film22 by a further ion implantation.

Specifically, as shown in FIG. 8A, first, an n-type impurity, arsenic inthis case, is ion implanted in a condition that an acceleration energyis 40 keV, a dose amount is 2.0×10¹⁵/cm², with the resist pattern 52 andthe gate electrodes 53 taken as masks, and the sources/drains 54 areformed at both sides of the gate electrodes 53 of the semiconductorsubstrate 1 corresponding to positions of the ONO film 15 exposed fromopenings 52 a of the resist pattern 52.

Next, as shown in FIG. 8B, the ion implantation of the arsenic isperformed in the condition that the acceleration energy is 10 keV to 20keV, and the dose amount is 1.0×10¹²/cm² to 2.0×10¹⁵/cm² from adirection inclined at a predetermined angle, a tilt angle of 45° in thiscase, with respect to the direction vertical to the surface of thesemiconductor substrate 1, with the resist pattern 52 taken as the maskagain, and lattice defects 22 a are selectively given to portionscorresponding to openings 52 a of the resist pattern 52 of the siliconnitride film 22, namely, upper portions of the sources/drains 54 of thesilicon nitride film 22 to form charge trap centers.

In the present embodiment, it is explained as an example that after theion implantation for forming the source/drain 54 is performed, the ionimplantation for giving the lattice defects 22 a to the silicon nitridefilm 22 is performed, however, the former ion implantation can beperformed after the latter ion implantation, reversing the order.

Subsequently, after the resist pattern 52 is removed by an ashingtreatment or the like, an n-type source/drain is formed at one elementactive region of the peripheral circuit region 12, and a p-typesource/drain (both not shown) is formed at the other element activeregion in the same way as the first embodiment.

After that, an interlayer insulating film, various kinds of contactholes, a wiring layer and the like are formed in the same way as thefirst embodiment to complete a MONOS transistor.

As described above, according to the present embodiment, a highlyreliable MONOS transistor can be realized, in which a high qualitysilicon nitride film 22 is formed by the plasma nitriding method in alow temperature condition, in addition, the silicon nitride film 22 canbe utilized as a charge trap film having a charge capture functionsufficiently adaptable for a miniaturization and a high integrationwhich are recent demands.

Third Embodiment

In the present embodiment, an embedded bit line type transistor having aSNOS structure as a semiconductor memory device will be disclosed. Notethat the structure of the transistor is explained with a manufacturingmethod thereof for convenience. With respect to structural members andthe like corresponding to the first embodiment, the same numerals aregiven and the explanations thereof are omitted.

A semiconductor memory device has such a structure that a SNOStransistor in a memory cell array region is a planar type and a CMOStransistor is formed in a peripheral circuit region thereof.

FIG. 9A to FIG. 9D are schematic sectional views showing the mainprocesses of the semiconductor memory device including the embedded bitline type SNOS transistor according to the present embodiment. In thepresent embodiment, an ON film 63 in which a silicon nitride film 61 anda silicon oxide film 62 are stacked is formed instead of forming the ONOfilm 15 in the first embodiment.

First, after respective processes of FIG. 1A and FIG. 1B are performedin the same way as the first embodiment and the silicon oxide film 2remained in the memory cell array region 11 is removed as shown in FIG.9A, a lower silicon oxide film 61 to be a tunnel insulating film and asilicon nitride film 62 to be a charge trap film respectively havingfilm thicknesses of approximately 7 μm, 10 μm are sequentially formed asthe ON film 63. The silicon nitride film 62 is formed by theaforementioned plasma nitriding method in the same way as the firstembodiment, and the lower silicon oxide film 61 is formed by a plasmaoxidizing, or a thermal oxidation method and a CVD oxidation method.

Subsequently, as shown in FIG. 9B, a photoresist is coated on the wholesurface and a resist pattern 16 is formed by a photolithography, ofwhich shape has openings only at portions where bit lines in the memorycell array 11 are formed.

Then, after bit line diffusion layers 17 are formed in the memory cellarray region by performing an ion implantation with the resist pattern16 taken as a mask, lattice defects are given to the silicon nitridefilm 62 by a further ion implantation.

Specifically, as shown in FIG. 9C, first, an n-type impurity, arsenic(As) in this case, is ion implanted in a condition that an accelerationenergy is 70 keV, and a dose amount is 2.0×10¹⁵/cm² with the resistpattern 16 taken as the mask to form the bit line diffusion layers 17 tobe sources/drains on a surface layer of the semiconductor substrate 1corresponding to positions of the ON film 63 exposed from the openings16 a of the resist pattern 16.

Next, as shown in FIG. 9D, the ion implantation of the arsenic isperformed in a condition that the acceleration energy is 10 keV to 20keV, and the dose amount is 1.0×10¹²/cm² to 2.0×10¹⁵/cm² from adirection inclined at a predetermined angle, a tilt angle of 45° in thiscase, with respect to the direction vertical to the surface of thesemiconductor substrate 1, with the resist pattern 16 taken as the maskagain, and lattice defects 62 a are selectively given to positionscorresponding to the openings 16 a of the resist pattern 16 of thesilicon nitride film 62, namely, the positions corresponding to upperportions of the bit line diffusion layers 17 of the silicon nitride film62 to form charge trap centers.

In the presents embodiment, it is explained as an example that after theion implantation for forming the bit line diffusion layers 17 isperformed, the ion implantation for giving the lattice defects 62 a tothe silicon nitride film 62 is performed, however, the former ionimplantation can be performed after the latter ion implantation,reversing the order.

After that, a SNOS transistor is completed through respective processescorresponding to the FIG. 2C to FIG. 3A, and FIG. 3B in the same way asthe first embodiment.

As described above, according to the present embodiment, a highlyreliable SNOS transistor can be realized, in which a high qualitysilicon nitride film 62 is formed by the plasma nitriding method in alow temperature condition, in addition, the silicon nitride film 62 canbe utilized as a charge trap film having a charge capture functionsufficiently adaptable for a miniaturization and a high integrationwhich are recent demands.

It is also preferable, in the present embodiment, in the same way as themodification example 1 of the first embodiment, as shown in FIG. 10,that the openings of the resist pattern 16 (openings 16 b) are enlargedwith a resist trimming performed to the resist pattern 16 after the bitline diffusion layers 17 are formed, and the lattice defects 62 a aregiven to the silicon nitride film 62 by an ion implantation with theresist pattern 16 taken as a mask.

Additionally, it is preferable that, in the same way as the modificationexample 2 of the first embodiment, as shown in FIG. 11A, after the bitline diffusion layers 17 are formed by the ion implantation with theresist pattern 16 taken as a mask, an ON film 63 is formed, subsequentlyas shown in FIG. 11B, the resist pattern 16 is removed and a resistpattern 51 is formed, and lattice defects 62 a are given to the siliconnitride film 62 by an ion implantation with the resist pattern 51 takenas a mask.

Furthermore, in the same way as the second embodiment, the presentinvention can be applied to a conventional transistor having a MNOSstructure, in which element isolation regions are formed in a memorycell array region and which has a channel vertical to a gate electrode,instead of an embedded bit line type transistor. In this case, as shownin FIG. 12A, after sources/drains 54 are formed in the memory cell array11 by an ion implantation with the resist pattern 52 and the gateelectrodes 53 taken as masks, lattice defects 62 a are given to thesilicon nitride film 62 of the ON film 63 by a further ion implantationas shown in FIG. 12B. Then, p-type and n-type sources/drains are formedin the peripheral region 12, and an interlayer insulating film, variouskinds of contact holes, a wiring layer and the like are formed tocomplete a semiconductor memory device.

Fourth Embodiment

In the present embodiment, an embedded bit line type transistor havingan SNS structure as a semiconductor memory device will be disclosed.Note that the structure of the transistor is explained with amanufacturing method thereof for convenience. With respect to structuralmembers and the like corresponding to the first embodiment, the samenumerals are given and the explanations thereof are omitted.

A semiconductor memory device has such a structure that an SNStransistor in a memory cell array region is a planar type and a CMOStransistor is formed in a peripheral circuit region thereof.

FIG. 13A to FIG. 13D are schematic sectional views showing the mainprocesses of the semiconductor memory device including the embedded bitline type SNS transistor according to the present embodiment.

In the present embodiment, a silicon nitride film 71 which is a singlelayer is formed instead of forming the ONO film 15 in the firstembodiment.

First, after the respective processes of FIG. 1A and FIG. 1B in the sameway as the first embodiment, the silicon oxide film 2 remained in thememory cell array region 11 is removed as shown in FIG. 13A, then, asilicon nitride film 71 having a film thickness of approximately 20 nmis formed by the aforementioned plasma nitriding method in the same wayas the first embodiment.

Subsequently, as shown in FIG. 13B, a photoresist is coated on the wholesurface and a resist pattern 16 is formed by a photolithography, ofwhich shape has openings only at portions where bit lines in the memorycell array region 11 are formed.

Then, after bit line diffusion layers 17 are formed in the memory cellarray region 11 by an ion implantation with the resist pattern 16 takenas a mask, lattice defects are given to the silicon nitride film 62 by afurther ion implantation.

Specifically, as shown in FIG. 13C, first, an n-type impurity, arsenic(As) in this case, is ion implanted in a condition that an accelerationenergy is 70 keV, and a dose amount is 2.0×10¹⁵/cm² with the resistpattern 16 taken as the mask to form the bit line diffusion layers 17 tobe sources/drains on a surface layer of the semiconductor substrate 1corresponding to positions of the silicon nitride film 71 exposed fromopenings 16 a of the resist pattern 16.

Next, as shown in FIG. 13D, the ion implantation of the arsenic isperformed in a condition that the acceleration energy is 10 keV to 20keV, and the dose amount is 1.0×10¹²/cm² to 2.0×10¹⁵/cm² from adirection inclined at a predetermined angle, a tilt angle of 45° in thiscase, with respect to the direction vertical to the surface of thesemiconductor substrate 1, with the resist pattern 16 taken as the maskagain, and lattice defects 71 a are selectively given to positionscorresponding to the openings 16 a of the resist pattern 16 of thesilicon nitride film 71, namely, the positions corresponding to upperportions of the bit line diffusion layers 17 of the silicon nitride film71 to form charge trap centers.

In the presents embodiment, it is explained as an example that after theion implantation for forming the bit line diffusion layers 17 isperformed, the ion implantation for giving the lattice defects 71 a tothe silicon nitride film 71 is performed, however, the former ionimplantation can be performed after the latter ion implantation,reversing the order.

After that, an SNS transistor is completed through respective processescorresponding to the FIG. 2C to FIG. 3A, and FIG. 3B in the same way asthe first embodiment.

As described above, according to the present embodiment, a highlyreliable SNS transistor can be realized, in which a high quality siliconnitride film 71 is formed by the plasma nitriding method in a lowtemperature condition, in addition, the silicon nitride film 71 can beutilized as a charge trap film having a charge capture functionsufficiently adaptable for a miniaturization and a high integrationwhich are recent demands.

It is also preferable, in the present embodiment, in the same way as themodification example 1 of the first embodiment, as shown in FIG. 14,that the openings of the resist pattern 16 (openings 16 b) are enlargedwith a resist trimming performed to the resist pattern 16 after the bitline diffusion layers 17 are formed, and the lattice defects 71 a aregiven to the silicon nitride film 71 by an ion implantation with theresist pattern 16 taken as a mask.

In addition, in the same way as the modification example 2 of the firstembodiment, it is preferable that after the bit line diffusion layers 17are formed by an ion implantation with the resist pattern 16 taken as amask before forming the silicon nitride film 71 as shown in FIG. 15A,the resist pattern 16 is removed and a resist pattern 51 is formed, andthe lattice defects 71 a are given to the silicon nitride film 71 by anion implantation with the resist pattern 51 taken as a mask as shown inFIG. 15B.

Furthermore, in the same way as the second embodiment, the presentinvention can be applied to a conventional transistor having an MNSstructure, in which element isolation regions are formed in the memorycell array region, instead of the embedded bit line type transistor. Inthis case, after sources/drains 54 are formed in the memory cell arrayregion by an ion implantation with a resist pattern 52 and gateelectrodes 53 taken as masks as shown in FIG. 16A, the lattice defects71 a are given to the silicon nitride film 71 by a further ionimplantation as shown in FIG. 16B. Then, p-type and n-typesources/drains are formed in the peripheral circuit region 12, and aninterlayer insulating film, various kinds of contact holes, a wiringlayer and the like are formed to complete an MNS transistor.

As described above, according to the present invention, a highlyreliable MNS transistor can be realized, in which a high quality siliconnitride film 71 is formed by the plasma nitriding method in a lowtemperature condition, in addition, the silicon nitride film 71 can beutilized as a charge trap film having a charge capture functionsufficiently adaptable for a miniaturization and a high integrationwhich are recent demands.

In the charge-storage memory composed of a single layer nitride film, aproperty as a gate electrode having less interface state between asemiconductor substrate is required with respect to the nitride film.Therefore, the device cannot form without using a plasma nitridingmethod in existing techniques.

Fifth Embodiment

In the first to fourth embodiments, the charge trap centers are formedby the ion implantation to the silicon nitride film to be a charge trapfilm, which is formed by the plasma nitriding method, however, in thepresent embodiment, an RF (radio frequency) processing using an inertgas, for example, an RF sputter etching is performed instead ofperforming the ion implantation.

As an example, a case including an embedded bit line type memory cellhaving a SNOS structure is explained in the same way as the thirdembodiment.

FIG. 17 is a schematic sectional view showing an RF process in amanufacturing method of a semiconductor memory device according to thepresent embodiment.

In the present embodiment, after the respective processes of FIG. 1A,FIG. 1B, and FIG. 9A to FIG. 9C in the same way as the third embodiment,the RF processing is performed using the inert gas such as argon (Ar) orxenon (Xe), an Ar gas in this case, with the resist pattern 16 takenagain as a mask, and lattice defects 62 b are selectively given toportions corresponding to openings 16 a of the resist pattern 16 of asilicon nitride film 62, namely, corresponding to upper portions of bitline diffusion layers 17 of the silicon nitride film 62 to form chargetrap centers.

It is desirable to perform the RF processing directly to the exposedsilicon nitride film in the manner of the present embodiment, notthrough a silicon oxide film and the like, because the energy of the RFprocessing is relatively small.

After that, a semiconductor memory device is completed throughrespective processes corresponding to the FIG. 2C to FIG. 3A, and FIG.3B in the same way as the second embodiment.

As described above, according to the present embodiment, a highlyreliable semiconductor memory device can be realized, in which the highquality silicon nitride film 22 is formed by the plasma nitriding methodin the low temperature condition, in addition, the silicon nitride film22 can be utilized as a charge trap film having a charge capturefunction sufficiently adaptable for a miniaturization and a highintegration which are recent demands.

In the above-described first to third embodiments and fifth embodiment,when the ONO film or the ON film is formed, it is explained by anexample that a surface layer of a silicon oxide film is directlyreplaced to a silicon nitride film by the plasma nitriding method,however, it is preferable that a silicon film is formed on the siliconoxide film in advance and the silicon film is plasma-nitrified.

To take the third embodiment as an example, as shown in FIG. 18B, afterthe lower silicon oxide film 61 to be a tunnel insulating film isformed, an amorphous silicon film 81 having a film thickness ofapproximately 5 nm is formed on the lower silicon oxide film 61, then,as shown in FIG. 18B, a silicon nitride film 82 to be a charge trapfilm, of which film thickness is approximately 10 nm is formed as an ONfilm 83. In this case, it is also preferable to form a polycrystallinesilicon film is formed instead of the amorphous silicon film.

Through the first to fifth embodiments, it is explained by examples thatthe lattice defects are given to the specified portions of the siliconnitride film formed by the aforementioned plasma nitriding method, andthe charge trap centers are formed in the specified positions, thepresent invention is not limited to this and the lattice defects can begiven, for example, to almost whole surface of the silicon nitride film,depending on the circumstances.

Non Patent Document 1

IEEE ELECTRON DEVICE LETTERS, VOL. 22, No. 11, NOVEMBER 2001“Characterization of Channel Hot Electron Injection by the SubthreshouldSlope of NROM™ Device” INDUSTRIAL APPLICABILITY

According to the present invention, a highly reliable semiconductormemory device can be realized, in which a high quality silicon nitridefilm is formed in a low temperature condition, in addition, the siliconnitride film can be used as a charge trap film having a charge capturefunction sufficiently adaptable for a miniaturization and a highintegration which are recent demands.

1. A semiconductor memory device, comprising: a semiconductor substrate;an insulating film formed on said semiconductor substrate and includinga nitride film having a charge capture function; a gate electrode formedon said semiconductor substrate through said insulating film; a pair ofimpurity diffusion layers formed on said semiconductor substrate, andwherein said nitride film includes charge trap centers with latticedefects formed at a specified portion of the nitride film.
 2. Thesemiconductor memory device according to claim 1, wherein said nitridefilm is a plasma nitride film of a densified structure formed by nothingbut a plasma nitriding by exiting a microwave or by a series ofprocesses including the plasma nitriding.
 3. The semiconductor memorydevice according to claim 1, wherein said insulating film is a stackedfilm in which said nitride film is formed on an oxide film.
 4. Thesemiconductor memory device according to claim 1, wherein saidinsulating film is a stacked film in which said nitride film issandwiched between respective upper and lower oxide films.
 5. Thesemiconductor memory device according to claim 1, wherein saidinsulating film is constituted by nothing but said nitride film.
 6. Thesemiconductor memory device according to claim 3, wherein said impuritydiffusion layer doubles as a bit line.
 7. The semiconductor memorydevice according to claim 4, wherein said impurity diffusion layerdoubles as a bit line.
 8. The semiconductor memory device according toclaim 5, wherein said impurity diffusion layer doubles as a bit line. 9.The semiconductor memory device according to claim 3, wherein a bit lineelectrically connected to said impurity diffusion layer is provided inan upper portion of said impurity diffusion layer.
 10. The semiconductormemory device according to claim 4, wherein a bit line electricallyconnected to said impurity diffusion layer is provided in an upperportion of said impurity diffusion layer.
 11. The semiconductor memorydevice according to claim 5, wherein a bit line electrically connectedto said impurity diffusion layer is provided in an upper portion of saidimpurity diffusion layer.
 12. A manufacturing method of a semiconductormemory device including: a semiconductor substrate; an insulating filmformed on said semiconductor substrate and including a nitride filmhaving a charge capture function; a gate electrode formed on saidsemiconductor substrate through said insulating film; and a pair ofimpurity diffusion layers formed on said semiconductor substrate,comprising a step of: forming charge trap centers by giving latticedefects to a part or the whole of the nitride film after the nitridefilm is formed by a plasma nitriding method.
 13. The manufacturingmethod of the semiconductor memory device according to claim 12, whereinthe charge trap centers are formed by introducing an impurity into thenitride film before or after the impurity diffusion layers are formed.14. The manufacturing method of the semiconductor memory deviceaccording to claim 12, wherein the lattice defects are given byperforming a radio frequency processing using an inert gas to thenitride film before or after the impurity diffusion layers are formed.15. The manufacturing method of the semiconductor memory deviceaccording to claim 12, wherein the nitride film is formed by performinga nitriding processing with the plasma excited by a microwave in anatmosphere of a source gas including nitrogen atoms for generating anitride radical.